Funds are requested for the acquisition of an ENDOR/TRIPLE system to be added to an existing ER 200 D X-band EPR spectrometer equipped with an Oxford cryostat for variable, low temperature operation. The instrument will be available on an interdepartmental basis to all qualified users in the biochemical, biophysical and biomedical sciences. Many biochemical processes revolve around paramagnetic centers and have long been studied by EPR. Important examples are electron transfer, radical reactions, catalysis by metalloenzymes, photosynthesis and photochemical reactions, which are under intensive investigation at the U of I. All these areas can greatly benefit from ENDOR, which allows to identify the magnetic moments of the nuclei in the paramagnetic center and to measure their hyperfine interactions. So far no such facility exists on this campus, and researchers in need of ENDOR spectroscopy have to collaborate with outside groups. Introduced in 1957 by G. Feher, Electron Nuclear Double Resonance yields NMR spectra of nuclei associated with paramagnetic centers. ENDOR has higher sensitivity but lower resolution then conventional NMR and can be applied to solids or liquids, subject to the condition that the EPR transition can be saturated. Because of its inherent complexity ENDOR has been applied relatively little to problems of biological interest. The new generation of commercial, high power instruments now on the market greatly facilitates the observation of ENDOR and makes it a practical, powerful tool for many investigations. The proposed spectrometer features an RF power of 300W from 0.2 - 35 MHz, computer control and signal averaging, and it allows simultaneous irradiation at two RF frequencies (TRIPLE resonance), variable temperature operation down to 3.5K, and easy exchange of samples. The proposed ENDOR facility complements the existing NMR and EPR facilities and will greatly enhance the research capabilities on this campus.